Allocations using `mul!`

[jlchan]

Thanks for this package! I've noticed there are some allocations when using mul!(y,A,x), where x,y are <: Vector and A is a KroneckerProduct. This seems to be due to alloc_temp_array in _kron_mul_fast_square!(C, B, factors).

Since we're calling mul! many times, these allocations add up. Would it make sense to have a version of KroneckerProduct which stores this temporary array as a hidden field to avoid allocating?

[MichielStock]

I think TensorOperations does something like that.

I am a bit hesitant to allow hidden matrices to be generated for this package, though it might make sense to have versions of mul! where users can provide a temp matrix?

[jlchan]

Thanks for the note on TensorOperations.jl. A version of mul! with a temporary array argument also makes sense. I'll take a look at TensorOperations.jl, but if Kronecker.jl fits better I'm happy to try a PR for the new mul!.

[MichielStock]

Kronecker is likely easier to use if you are just doing matrix algebra. TO is a bit more general and advanced and can likely attain somewhat higher performance. Take a look and share your thoughts if we should extend Kronecker in this direction.

[jlchan]

After looking at TO, I think Kronecker.jl would be a better fit. I can work on a PR for the temporary array argument mul!, or if you have some ideas I'm happy to try to implement them.

[MichielStock]

Maybe you can best take the lead in this and we can look together if it can be improved. Feel free to open a PR!